Heat exchanger for transferring heat between gases

ABSTRACT

A heat exchanger for taking heat from a hot gas and delivering the heat to a cool gas. The heat exchanger includes a lower passage structure through which the hot gas flows and an upper passage structure through which the cool gas, which is to be heated, flows. A heat-transfer structure is operatively connected with the upper and lower passage structure so as to be in heatexchanging relation therewith, this heat-transfer structure including an elongated vertical enclosure which is in heattransfer communication with the gas in the lower passage structure as well as with the gas in the upper passage structure. This heat-transfer structure includes within the latter enclosure a fluid which is in a liquid phase in a lower portion of the enclosure which is operatively connected with the lower passage structure and in a vapor phase in an upper portion of the enclosure which is operatively connected with the upper passage structure. Thus, heat from the hot gas in the lower passage structure serves to vaporize the liquid phase of the fluid in the enclosure with the vapor rising up to the elevation of the upper passage structure where the vapor gives up its heat to the gas in the upper passage structure with consequent condensing of the vapor to form again the liquid phase which settles to the lower portion of the enclosure.

Heller et al.

. HEAT EXCHANGER FOR TRANSFERRING HEAT BETWEEN GASES [75] Inventors: Ltiszlo Heller; Liszlo Sziics; Zoltn Szab, all of Budapest, Hungary [73] Assignee: Energiagazdalkodasi lntezet,

Budapest, Hungary 22 Filed: Sept. 16, 1971 [2l] Appl. No.: 181,125

[30] 1 Foreign Application Priority Data 506,396 10/1954 Canada 165/165 Primary Examiner-Albert W. Davis, Jr. Attorney, Agent, or Firm-Blum Moscovitz Friedman & Kaplan 1451 May 7, 1974 [57] ABSTRACT A heat exchanger for taking heat. from a hot gas and delivering the heat to a cool, gas. The heat exchanger includes a lower passage structure through which the hot gas flows and an upper passage structure through which the cool gas, which is to be heated, flows. A heat-transfer structure, is operatively connected with the upper and lower passage structure so as to be in heabexchanging relation therewith, this heat-transfer structure including an elongated vertical enclosure which is in heat-transfer communication with the gas in the lower passage structure as well as with the gas in the upper passage structure. This heat-transfer structure includes within the latter enclosure a fluid which is in a liquid phase in a lower portion of the enclosure which is operatively connected with thelower passage structure and in a vaporphase in an upper portion of the enclosure which is operatively connected with the upper passage structure. Thus, heat from the hot gas in the lower passage structure serves to vaporize the liquid phase of' the fluid in the enclosure with the vapor rising up to the elevation of the upper passage structure where the vapor gives up its heat to the gas in the upper passage structure with consequent condensing of the vapor to form again the liquid phase which settles to the lower portion of the enclosure.

5 Claims, 2 Drawing Figures HEAT EXCHANGER FOR TRANSFERRING HEAT BETWEEN GASES BACKGROUND OF THE INVENTION power plants, the use of heat exchangers is of great importance. Such heat exchangers bring about an exchange ofheat in the recuperator of the power plant, and it is desirable that the operation of such a recuperator be inexpensive.

When dealing with .transfer of heat between gases, an extremely large surface area is usually required in the heat exchanger because of the fact that the heattransfer properties of gases are not favorable to exchange of heat. Thus, in those cases where the heattransfer "properties of the fluids which are in heatexchanging relation are unfavorable, it is conventional to apply in'the passages through which the fluids flow projections, fins, etc., which will greatly. increasethe surface area of the heat exchanger whichis exposed to the gas. The use of projections ofthis latter type will in most cases result in a decrease in the cost of the heat exchanger.

However, experience has shown that 'this type of heat exchanger has notproved to be sufficiently economical. Thus, in the transfer of heat between gases,the complex structureswhich, are required to provide the large surface area must be applied to bothgases, with the result that considerable technical difficulties are encountered in view of the connections which are required between the heat-transferring bodies with their large surface areas, particularly if requirements with respect to a relatively high efficiency of the heat transfer, or in other words an efficient utilization factor of the heat exchanger, is maintained. For example in the case of recuperators used with closed cycle gas turbines, these latter requirements are essentialfor economical operation. The difficulties which are encountered are also present with respect to the layout of the entire heat exchanger, particularly at such areas as where the gas isintroduced into the heat exchanger and where it is withdrawn therefrom,-and magnitude of the difficulties rises with an increase in the amount of heat which is to be transferred. For example in the case of recuperators of closed cycle turbines having a nuclear heat source the heat to be transferred is of an extremely large magnitude (on the order of 1,000-2,500 MW,).

Under these latter conditions the use of complex surface structures which will give the required large surface area results in an undesirable increase in the pressure drop of the gases, with consequent disadvantages referred to in greater detail below.

Because of the above factors, the'use of complex heat-exchanger structures having projections, fins, and the like, is dispensed with in closed cycle gas turbine power plants having a nuclear heat source, and instead use is made almost exclusively of pipes which have smooth surfaces. (See Burgemuller P, Roduner l-I., Der

2 Rekuperator fur einen schnellen Leistungsreaktor von 1,000 MW mit direktem Gasturbinenkreislauf, BWK 21 (1969) Nr. 10. resp. Rammert, K. K'lakens, H. Mukherjee, S. K. Auslegung und Konstrucktion von Warmetauschern fur geschlossene Gasturbinenanlagen,

BWK 22 (1970) Nr. 6. However, this latter type of solution to the problem has the great disadvantage of requiring extremely large dimensions for the heat exchanger. The result is not only an increase in cost of the structure but also the cost of the building 'where the structure is set up is undesirably increased, with the resulting extremely large costs involved in the case of integrated or semi-integrated building structures.

SUMMARY OF THE INVENTION It is accordingly a primary object of the present invention to provide a heat exchanger which will avoid the above drawbacks. More specifically, it is'an object of the invention -to provide a heat exchanger which will have relatively small dimensions and occupy a relatively small space as compared to heat exchangers of conventional structure which can accomplish the same results as the heat exchanger of the present invention.

Furthermore, it is an object of the present invention to provide a heat exchanger which is particularly suitable for closed cycle gas turbines having a nuclear heat source.

Also it is an object of the present invention to provide a heat exchanger where the pressure drop is only a relatively small fraction of the pressure drop now encoun tered in heat exchangers designed to accomplish results similar to those achieved with the heat exchanger of the invention.

According to the invention the heat exchanger includes a lower passage means for conducting the hot gas from which heat is to be derived and, over the lower passage means, an upper passage means for conducting the cool gas which is to be heated with the heat taken from the hot gas. A heat transfer means extends between and is operatively connected with the lower and the upper passage means for transferring heat from the gas in the lower passage means tothe gas in the upper passage means, and this heat transfer means includes an elongated enclosure having a lower portion in heatexchanging relation with the lower passage means and an upper portion in heat-exchanging relation with the upper passage means. Within this enclosure the heattransfer means includes a fluid which is in its liquid phase in the lower portion of the enclosure and in its vapor phase in the upper portion of the enclosure. The fluid is converted from its liquid to its vapor phase with heat derived from the gas in the lower passage means, and the vapor which is thus formed rises to the upper portion of the enclosure where it gives up heat to the gas in the upper passage means while simultaneously cooling and condensing to form again the liquid phase which settles to the lower portion of the enclosure. Ac

BRIEF DESCRIPTION OF DRAWINGS DESCRIPTION OF A PREFERRED EMBODIMENT As is well known, the density or magnitude of heat output (cal/hr,m at a given temperature difference can be increased, in the case of heat exchangers having smooth-surface pipes, only at the cost of a pressure drop in the gases. Inasmuch as 1 percent of the relative increase in the gas pressure drop results in approximately 0.5 percent loss in efficiency (see Bammert, K., Twardzick, W. 'Kernkraftwerke mit Heliumturbinen fur grosse Leistungen, Atomenergie, 1967. H9/l0). The total pressure drop in a given installation is great enough (on the order of 3.5 percent) so that a further increase in the pressure drop does not provide an effective favorable solution to the problem.

Of course it is highly desirable to achieve an increase in heat output with a given temperature differencein such a way that not only is there no increase in the pressure drop but in fact there is a decrease in the pressure drop, and this outstanding result is achieved with the present invention.

The present invention is based upon the fact that if a fluid which is in its liquid phase is placed in heatexchanging relation with a hot gas, the fluid will have its liquid phase vaporized into a vapor phase which can give up its heat to a cool gas which is to be heated with consequent condensing of the vapor phase back into the liquid phase of the fluid. By utilizing this latter principle it is possible with the present invention to use for both the hot gas and the cool gas relatively large areas at the heat-transfer surfaces, without, however, resorting to complex surfaces having projections, fins, and the like.

The use of an intermediate heat-transfer fluid results, however, in the requirement that the heat has to be transferred twice, namely first from the hot gas to the heat-transferring fluid and then from the latter to the cool gas, even with a relatively small temperature difference between the gases which flow through the heat exchanger, so that it is essential at least theoretically that at least four or five times the surface area be exposed to the gases than would otherwise be required. As a result of these considerations, the objective of increasing the heat output while decreasing the pressure drop cannot be achieved economically with conventional surfaces in the heat exchanger, as has indeed been demonstrated by failures of attempts to achieve all types of heat exchangers because. of the danger of fouling and clogging of the passages with deposits from the gases. However, in the particular application of the present invention to close cycle gas turbines with nuclear reactors, there is no danger of fouling or clogging of the narrow passages, and therefore with the present invention use is made of extremely narrow gas passages providing a substantially laminar gas flow with a particular limit to the depth of the gas passages which would not be suitable for general use. Thus, according to the invention the extremely narrow gas passages will include at least some gas passages where the depth is no greater than 0.7 mm.

The hot and cold gases are directed through these extremely narrow passages which are in heat-exchange communication with enclosures in which the heattransferring fluid is located, and the fluidin this enclosure is chosen in such a way that the heat from the hot gas will change the fluid from its liquid phase to its vapor phase while the vapor phase will be converted back to the liquid phase during transfer of heat to the cold gas so that the particular fluid chosen will have a saturation temperature which will be adapted to the particular properties of the gases which flow through the passages. Thus, the particular physical properties of the fluid in the enclosure of the heat-transfer means, such as the saturation pressure, density, viscosity, evaporation and condensing heat of the fluid will have an effect on the heat transfer and strength characteristics and will be adapted to the particular temperature differential which is encountered. Thus, when the hot gas initially enters the heat exchanger it has a relatively high temperature while where it leaves the exchanger it has a relatively low temperature, and of course the cold gas leaves the exchanger at a relatively high temperature and enters at a relatively low temperature, so that along the paths of the hot and cold gases'there are a series of enclosures with heat-transfer fluids therein, and the particular fluids in the different enclosures will be different in accordance with whether the fluid is located at that part of the apparatus where the hot gas initially enters and the cold gas leaves or whether it is at part of the apparatus where the hot gas leaves the exchanger and the cold gas enters. Thus, by utilizing different fluids in the different enclosures of the heattransfer means it is possible to provide the most suitable fluid for the particular heat exchange which is taking place at a particular part of the apparatus. Thus, for example, where the hot gas enters the apparatus the fluid may be mercury while where the hot gas leaves the apparatus the fluid may be water.

It is also desirable to arrange the passages of the pair of passage means for the gases in such a way that they form columns while the enclosures are arranged in spaces between the columns, so that in this way it is possible to achieve an extremely compact structure which is economical. By arranging a series of fluid enclosures between groups of passages which form columns, it is possible with a given surface area and heat load to diminish the temperature difference between the pair of gases. Referring to the drawings, the heatexchanger structure of the invention illustrated therein includes an upper passage means 1 for the cold gas and a lower passage means 2 for the hot gas. The lower and upper passage means 2 and 1 are maintained separate from each other through suitable baffles and suitable headers or the like direct the cold gas through the passages of the upper passage means 1 in the direction of the arrow shown at the lower left of FIG. 2 while the hot gas flows through the passages of the lower passage 7 means 2 in the opposite direction as indicated by the arrows at the upper right portion of FIG. 2. The hot gas is designated with the dotted areas while the cool gas is designated with the clear areas.

As is apparent from the drawings each of the passage means 1 and 2 includes a body of good thermoconductivity formed with groups of elongated narrow passages which have a relatively small depth in relation to their width. In effect the passages of both of the passage means 1 and 2 form elongated extremely narrow slots some of which at least have a depth which is no greater than 0.7 mm. These slots or passages of the pair of passage means 1 and 2 are arranged in groups which form columns in that the several passages of the lower passage means 2 are arranged in groups situated one above the other as illustrated in FIG. 1, while situated directly over the slots or passages of the lower passage means 2 are corresponding groups of identical passages which form the passages for the upper passage means 1. In this way a series of columns of passages are formed with the groups of passages which form the several columns being separated from each other. In the space between these columns or groups of passages are located the vertical enclosures 3 of the heat-transfer means. Thus, between each pair of successive groups of passages there are a series of vertically extending enclosures 3 which arearranged in a series along the path of gas flow, as is apparent from FIG. 2 in particular.

Within each enclosure 3 the heat-transfer means includes a heat-transfer fluid 4 which in a lower portion of each enclosure whichis in heat-exchanging relation with the lower passage means 2 has the fluid 4 in its liquid phase while in the upper portion which is in heatexchanging relation with the upper passage means 1 the fluid in each enclosure is in its vapor phase. While it is possible to provide different sizes for the several passages, they all may have the same small size referred to above. Thus, the series of vertically arranged horizontal passages of each group of passagesof the lower passage means 2 and the upper passage means 1 may beconsidered as forming a single column 5 of passages, and it will be noted that the elongated enclosures 3 extend perpendicularly to the passages.

The above-described structure of the invention operates as follows:

The heat exchanger is ready for operation when the lower passage means 2 for the hot gas is situated beneath the upper passage means 1 for the cold gas. The elongated enclosures 3 form a coupling between the lower and upper passage means, being in heatexchanging relation therewith through the wall of the body which is formed with the passages and enclosures.

Assuming now that the flow of, hot gas is started through the lower passage means 2, then the fluid 4 which is in its liquid phase in the lower portions of the enclosures 3 will boil toform the vapor which rises into the upper portions of the enclosures 3 where the vapor accumulates under these conditions.

If at thesame time the flow of cold gas is started, the vapors which rise to the upper portions of the enclosures 3 will condense while transferring heat to the gas in the passage means 1, thus returning from the vapor to the liquid phase and settling back to the lower portion'of the enclosures 3, with these operations being continuously carried out.

Thus, the heat which is delivered from the vapor will be absorbed by the cold gas in the passage means 1, while the liquid phase of 'the heat-transferring fluids will be heated with the hot gas in the lower passage means 2. Thus, whatever liquid evaporates in each enclosure 3 is subsequently made upby the condensate which precipates gravitationally back down to the lower portion of each enclosure 3, the condensate formingdroplets or the like which settle on the inner surfaces of the enclosures 3 and flow down to the bottom thereof. j

The invention has been carried out with'the series of enclosures 3 arranged as shown in FIG. 2 between each pair of successive groups of passages of the lower and upper passage means 2 and l, and different fluids were placed in different enclosures 3. in accordance with the situation thereof in the heat exchanger. Thus the characteristics of the particular fluid at a particular location will be adapted to the particular temperature prevailing at the particular part of the heat exchanger.

It has been found that with the above-described structure of the invention the'problems encountered in conventional structures have been effectively solved.

Thus the structure of the invention is particularly suit-' able as a recuperator for a closed cycle gas turbine having a nuclear heat source. The volume occupied by the entire heat exchanger of the invention is only about one third of the volume required by a conventional heat ex changer which will have a similar output. Furtheremore, the pressure drop is only about one seventh of the volume or pressure drop now encountered in the most modern heat exchangers which are designed to operate under the same conditions as those encountered with the heat exchanger of the invention.

Whatis claimed is:

1. In a heat exchanger, lower passage means for conducting a hot gas and upper passage means for conducting a cool gas which is to be heated with heat derived from the hot gas flowing through the lower passage means, and heat-transfer means operatively connected with said lower and upper passage means for taking heat from the gas in the lower passage means and delivering the heat to the gas in the upper passage means, said heat-transfer means including an elongated substantially vertical enclosure having a lower portion at the elevation of and operatively connected with said lower passage means and an upper portion at the elevation of and operatively connected with said upper passage means, said heat-transfer means including within said enclosure a fluid which is in its liquid phase in said lower portion and in its vapor phase in said upper portion of said enclosure, said liquid phase of said fluid being converted into a vapor with heat from the gas in said lower passage means and said vapor rising to the upper portion of said enclosure to give up heat to gas in said upper passage means while condensing to form again the liquid phase which settles to the lower portion of said enclosure, said lower'passage means including an elongated body formed with a plurality of substantially horizontally extending passages each of which has anextremely small depth and a width which is much greaterthan its depth and said passages of said lower passage means being arranged one above the other, said upper passage means having a construction substantially identical with said lower passage means and being situated directly thereover, and saidheat-transfer means having said enclosure thereof completely closed and extending perpendicularly with respect to the passages.

2. The combination of claim 1 and wherein each of said passage means includes at least one body formed with a plurality of substantially horizontally extending passages through which the gas flows, and each of said passages having a width which is substantially greater than its depth.

3. The combination of claim 2 and wherein said depth of each of said passages is no greater than 0.7

4. The combination of claim 1 and wherein said heattransfer means includes a plurality'of said enclosures arranged in a series one next to the other along said lower and upper passage means and someof said enclosures having fluids different from the fluids in other enclosures with the different fluids being adapted to the particular temperatures of the gases at the locations of the enclosures. I

5. The combination of claim 1 and wherein said lower passage means includes a plurality of groups of horizontal passages with the passages in each group arranged one above the other and with the several groups spaced from each other with the passages all extending parallel to each other while said upper passage means includes corresponding groups of upper passages respectively arranged over the groups of the lower passage means, and the heat-transfer means including between each pair of groups of said upper and lower passage means a series of vertically arranged enclosures situated one next to the other and separating one group of passages from the next. 

1. In a heat exchanger, lower passage means for conducting a hot gas and upper passage means for conducting a cool gas which is to be heated with heat derived from the hot gas flowing through the lower passage means, and heat-transfer means operatively connected with said lower and upper passage means for taking heat from the gas in the lower passage means and delivering the heat to the gas in the upper passage means, said heat-transfer means including an elongated substantially vertical enclosure having a lower portion at the elevation of and operatively connected with said lower passage means and an upper portion at the elevation of and operatively connected with said upper passage means, said heat-transfer means including within said enclosure a fluid which is in its liquid phase in said lower portion and in its vapor phase in said upper portion of said enclosure, said liquid phase of said fluid being converted into a vapor with heat from the gas in said lower passage means and said vapor rising to the upper portion of said enclosure to give up heat to gas in said upper passage means while condensing to form again the liquid phase which settles to the lower portion of said enclosure, said lower passage means including an elongated body formed with a plurality of substantially horizontally extending passages each of which has an extremely small depth and a width which is much greater than its depth and said passages of said lower passage means being arranged one above the other, said upper passage means having a construction substantially identical with said lower passage means and being situated directly thereover, and said heat-transfer means having said enclosure thereof completely closed and extending perpendicularly with respect to the passages.
 2. The combination of claim 1 and wherein each of said passage means includes at least one body formed with a plurality of substantially horizontally extending passages through which the gas flows, and each of said passages having a width which is substantially greater than its depth.
 3. The combination of claim 2 and wherein said depth of each of said passages is no greater than 0.7 mm.
 4. The combination of claim 1 and wherein said heat-transfer means includes a plurality of said enclosures arranged in a series one next to the other along said lower and upper passage means and some of said enclosures having fluids different from the fluids in other enclosures with the different fluids being adapted to the particular temperatures of the gases at the locations of the enclosures.
 5. The combination of claim 1 and wherein said lower passage means includes a plurality of groups of horizontal passages with the passages in each group arranged one above the other and with the several groups spaced from each other with the passages all extending parallel to each other while said upper passage means includes corresponding groups of upper passages respectively arranged over the groups Of the lower passage means, and the heat-transfer means including between each pair of groups of said upper and lower passage means a series of vertically arranged enclosures situated one next to the other and separating one group of passages from the next. 